Dual-loop control for laser annealing of semiconductor wafers

ABSTRACT

Systems and methods for performing semiconductor laser annealing using dual loop control are disclosed. The first control loop operates at a first frequency and controls the output of the laser and controls the 1/f laser noise. The second control loop also controls the amount of output power in the laser and operates at second frequency lower than the first frequency. The second control loop measures the thermal emission of the wafer over an area the size of one or more die so that within-die emissivity variations are average out when determining the measured annealing temperature. The measured annealing temperature and an annealing temperature set point are used to generate the control signal for the second control loop.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of co-pending U.S. patentapplication Ser. No. 13/706,397, filed on Dec. 6, 2012, from whichpriority is claimed and which is incorporated by reference herein.

FIELD

The present disclosure relates generally to laser annealing ofsemiconductors, and in particular relates to dual-loop control forsemiconductor laser annealing.

BACKGROUND ART

Laser annealing (also called laser spike annealing or laser thermalprocessing) is used in semiconductor manufacturing for a variety ofapplications, including for activating dopants in select regions ofdevices (structures) formed in a semiconductor wafer when forming activemicrocircuits such as transistors.

One form of laser annealing uses a scanned laser beam (“laser annealingbeam”) to heat the surface of the wafer to a temperature (the “annealingtemperature”) for a time long enough to activate the dopants in thesemiconductor structures (e.g., source and drain regions) but shortenough to prevent substantial dopant diffusion. The time that the wafersurface is at the annealing temperature is determined by the powerdensity of the laser annealing beam, as well as the exposure time, whichis given by the width of the beam along the scan direction divided bythe velocity at which the laser annealing beam is scanned (the “scanvelocity”).

Typical semiconductor processing requirements call for the annealingtemperature to be between 400° C. and 1,300° C., with a temperatureuniformity of +/−3° C. To achieve this degree of temperature uniformity,the laser annealing beam needs to have a relatively uniform intensity inthe cross-scan direction, which under most conditions represents lessthan a +/−5% intensity variation.

However, even when the laser annealing beam is made spatially veryuniform, feedback to the laser is required to ensure that the annealingtemperature remains uniform to within the stated tolerance. Localemissivity variations on patterned wafers can cause a temperaturemeasurement error when the system cannot distinguish between a change inemission due to true temperature change and a local change inemissivity. For most logic device wafers, the thickness and compositionof the patterned regions is such that the deviation in patternemissivity from bulk silicon is relatively small.

For other types of device wafers, the variations in emissivity can besubstantial. For example, memory wafers have thick metal lines. Also,certain logic wafers include a silicide step wherein the patternedregions have a relatively thick metal-silicide (e.g., NiSi). In both ofthese cases, the variation in thermal emission from the emissivityvariations is large. Consequently, as the laser annealing beam scanssuch wafers, the amplitude and time-frequency of the pattern-inducedemission variation is such that the temperature control system canbecome unstable.

Re-tuning the temperature control system to respond to emission spikeswould cause the laser power to be modulated in response to emissivityvariations, and not temperature variations. As a result, the laserannealing system must process silicide and memory wafers in open loop(constant laser power) condition, which limits the temperatureuniformity performance of the laser anneal due to the uncompensatedeffects such as laser power density fluctuation and/or variation in thelocal substrate temperature.

This in turn limits the maximum safe annealing temperature. Theannealing temperature needs to be kept below the damage thresholdtemperature of the wafer. The wider distribution of anneal temperaturesunder open-loop processing requires a reduction in the mean annealtemperature to keep the extremes of the anneal temperature distributionbelow the damage threshold. This presents a process compromise when (asis the case in most spike anneal process applications) a higher annealtemperature (below damage threshold) produces a superior process result

SUMMARY

An aspect of the disclosure is a method of laser annealing a waferhaving a surface that supports an array of dies. The method includesscanning an annealing laser beam from the laser over the array of dies,wherein the laser has laser noise and is adjustable to control an amountof power in the annealing laser beam. The method also includes measuringand controlling the amount of power in the annealing laser beam using afirst control loop that measures the amount of power and operates at afirst frequency f₁ to control the laser noise in the laser. The methodfurther includes controlling the amount of power in the annealing laserbeam using a second control loop that operates at a second frequencyf₂<f₁ by measuring thermal emission radiation from the wafer, includingaveraging the thermal emission radiation over at least one die anddetermining therefrom a corresponding average measured temperature, andusing the average measured temperature and an annealing temperature setpoint to adjust the laser to control the amount of power in theannealing laser beam.

Another aspect of the disclosure is a laser annealing system forannealing dies supported by a wafer having a surface that supports dieseach having a variation in emissivity. The system includes a chuck thatsupports the wafer, and a movable stage that supports the chuck and thatis adapted to move the chuck and wafer. The system also has a laser thatgenerates an initial laser beam and that has an adjustable output powerand 1/f noise. The system further includes an optical system configuredto receive the initial laser beam and form therefrom an annealing laserbeam and to direct a portion of the annealing laser beam to aphotodetector system that in response generates a detector signal. Theoptical system is arranged so that the annealing laser beam is madeincident upon and scans over the wafer surface due to the movement ofthe wafer, and heats the wafer to an annealing temperature. The systemalso includes a thermal emission detector system arranged relative tothe wafer surface and configured to receive thermal emission radiationtherefrom that is generated by the annealing laser beam and average thereceived thermal emission radiation over an area of one or more of thedies, and in response generates a thermal emission signal. The systemalso has a first control loop configured to receive the detector signaland control the laser to adjust the output power at a first frequency f₁in the range from 1 kHz to 100 kHz to reduce the 1/f noise. The systemfurther has a second control loop configured to receive the thermalemission signal and control the laser to adjust the output power at asecond frequency f₂ in the range from 1 Hz to 100 Hz to reducevariations in the annealing temperature.

Another aspect of the disclosure is a dual-loop control system for alaser annealing system that uses an annealing laser beam from a laserhaving 1/f noise and a controllable output power to anneal a waferhaving dies that each have a variation in emissivity. The system hasfirst and second control loops. The first control loop operates at firstfrequency f₁ in a range between 1 kHz and 100 kHz and is configured tocontrol the laser output power to reduce the 1/f laser noise. The secondcontrol loop operates at a second frequency f₂ in a range between 1 Hzand 100 Hz and measures thermal emission radiation from the wafer overan area of one die or greater. The thermal emission is caused by theannealing laser beam heating the wafer to an annealing temperature. Thesecond control loop is configured to determine a measured wafertemperature from the measured thermal emission radiation and generate ameasured temperature signal. The measured temperature signal is thenused in combination with an annealing temperature set point signal tocontrol the laser output to minimize variations in the annealingtemperature.

Additional features and advantages of the disclosure are set forth inthe detailed description that follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the disclosure as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following Detailed Description present embodiments of thedisclosure, and are intended to provide an overview or framework forunderstanding the nature and character of the disclosure as it isclaimed.

The claims are incorporated into and constitute part of the DetailedDescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure, and are incorporated into andconstitute a part of this specification. The drawings illustrate variousembodiments of the disclosure, and together with the description serveto explain the principles and operations of the disclosure. In certainof the drawings, Cartesian coordinates are provided for reference andare not intended to be limiting as to direction and orientation.

FIG. 1 is a schematic diagram of an example laser annealing system thatillustrates an embodiment of the dual-loop control system for the laserannealing system when performing laser annealing of a semiconductorwafer; and

FIG. 2 is a plan view of an example wafer showing the line image formedby the annealing laser beam scanning over the laser surface, andincluding an inset that shows the wafer having an array of dies formedon the wafer surface.

DETAILED DESCRIPTION

Reference is now made in detail to embodiments of the disclosure,examples of which are illustrated in the accompanying drawings. Wheneverpossible, the same or like reference numbers and symbols are usedthroughout the drawings to refer to the same or like parts.

FIG. 1 is a schematic diagram of an example laser annealing system 10that illustrates an embodiment of the dual-loop control configurationfor the system. A Cartesian coordinate system is shown for the sake ofreference. Example laser annealing systems 10 are described, forexample, U.S. Pat. Nos. 7,612,372, 7,154,066 and 6,747,245, and U.S.Patent Application Publications No. 2010/0084744 and 2012/0100640, allof which are incorporated by reference herein.

System 10 includes laser 20 that emits an initial laser beam 22 along anoptical axis A1 that runs in the X-direction. An example laser 20 is aCO₂ laser that emits light at a nominal annealing wavelength λ_(A) of10.6 μm. Laser 20 may also consist of or include one or more diodelasers.

System 10 also includes an optical system 30 arranged along optical axisA1 downstream of laser 20. Optical system 30 is configured to receiveinitial laser beam 22 and form therefrom a laser annealing beam (alsoreferred to below as annealing laser beam) 24. Example optical system 30can include lenses, mirrors, apertures, filters, active optical elements(e.g., variable attenuators, etc.) and combinations thereof. Examples ofoptical systems 30 are disclosed in U.S. Pat. Nos. 8,026,519, 8,014,427,7,514,305, 7,494,942, 7,399,945 and 6,366,308, all of which areincorporated by reference herein, and are also disclosed in theaforementioned U.S. Patent Publication No. 2012/0100640.

In an alternate embodiment of system 10, a second laser and a secondlaser beam (not shown) are used to enhance the annealing process, e.g.,by pre-heating the wafer.

Also arranged along optical axis A1 downstream of optical system 30 is abeam-turning element 34, such as a mirror having a surface 36 that issubstantially reflective at the annealing wavelength λ_(A). This may beaccomplished, for example, by making beam-turning element 34 out ofcopper and providing surface 36 as a gold coating. In an example,beam-turning element is part of optical system 30, and is shown as beingseparate by way of example and for ease of illustration and discussion.

Beam-turning element 34 defines a second optical axis A2 that isdirected toward a semiconductor wafer 40 having a top surface(“surface”) 42 with a surface normal N. Wafer 40 is supported by a chuck50 (e.g., a heated chuck), which in turn is supported by a moveablewafer stage 52 that is driven by a stage driver 54. Optical axis A2defines an incident angle θ2 on wafer surface 42 relative to surfacenormal N. Laser annealing beam 24 travels down optical axis A2 andintersects wafer surface 42. Wafer 40 has an overall wafer temperatureT_(W), which in an example is determined by heated chuck 50, and whichin an example is at least greater than room temperature.

In an example, the intersection of annealing laser beam 24 with wafersurface 42 defines a laser beam line or “line image” 24L that scans overwafer surface 42. The scanning of line image 24L can accomplished bymoving stage 52 by the operation of stage driver 54, by adjustingbeam-turning element 34 to cause the annealing laser beam to be scanned,or a combination of these two actions.

As discussed above, typical semiconductor processing requirements callfor the annealing temperature T_(A) to be between 400° C. and 1,300° C.,with a temperature uniformity of +/−3° C. To achieve this degree oftemperature uniformity, annealing laser beam 24 needs to have arelatively uniform intensity in the cross-scan direction, which undermost conditions is less than a +/−5% intensity variation.

In an example, the maximum temperature for laser spike annealing is atleast about 25° C. below the melt temperature of the material beingannealed. In the case of silicon-based devices, the melt temperature ofsilicon is 1413° C. so that in an example the maximum temperature willbe about 1388° C.

In the case of germanium-enriched silicon, the melt temperature of thealloy depends on the percentage of Ge in the alloy. For 30% Ge, the melttemperature is approximately 1225° C., so that an example maximumtemperature for laser annealing will be about 25° C. lower, or about1200° C.

In the case of GaN on sapphire devices, the melt temperature of GaN isover 2500° C., but the melt temperature of the Sapphire substrate islower at 2040° C. Thus, the non-melt annealing process is limited by themelt temperature of the sapphire substrate, and in an example themaximum anneal temperature about 2015° C.

FIG. 2 is a plan view of an example wafer 40 that includes a close-upinset that shows the wafer surface 42 supporting an array of dies 44each separated by a kerf region 46. Dies 44 represent regions where oneor more integrated circuit (IC) chips are formed, with each IC chipcomprising semiconductor devices and semiconductor device structures(not shown). In one example, wafer 40 is a logic wafer used to makelogic IC chips, while in another example the wafer is a memory waferused to make memory IC chips.

As discussed above, the semiconductor devices and semiconductor devicestructures within dies 44 can have features with different emissivities,which can lead to the miscalculation of annealing temperatures duringthe annealing process when trying to measure annealing temperatureswithin a given die. For example, as mentioned above, memory wafersusually have thick metal lines. Also, logic wafers can include asilicide step wherein the patterned regions have a relatively thickmetal-silicide (e.g., NiSi) layer. Metal lines and metal-silicide havemuch lower emissivities than the underlying silicon.

In FIG. 2, the line image 24L is shown as being scanned back and forthover wafer surface 42 to cover adjacent rows of dies 44. The scanning isindicated by arrows AR1 and AR2, which correspond to adjacent scan pathsSP1 and SP2 for line image 24L. Scanning of the line image 24L acrossthe wafer surface 42 may be accomplished by using the moveable waferstage 52 to scan wafer surface under a stationary line image 24L or byscanning the line image 24L across a stationary wafer surface 42 by useof appropriate scanning optical elements as part of optical system 30 orthe beam-turning element 34. By extension, a combination of these twoapproaches may be used.

In an example, surface 36 of beam-turning element 34 is configured todeflect a relatively small portion 24P of laser annealing beam 24 alongan optical axis A3. In an example, this is accomplished by providing aweak (e.g., lightly scribed) diffraction grating 36G onto surface 36. Aphotodetector system 60 is arranged along optical axis A3 and isarranged to receive laser annealing beam portion 24P and in responsethereto generate a detector signal SD representative of the measuredpower. In an example, the amount of power in laser annealing beamportion 24P is only a few percent of the power in laser annealing beam24. Whether by design or by calibration, the amount of power inannealing laser beam portion 24P is a known fraction of annealing laserbeam 24, so that a measurement of the annealing laser beam portionprovides a measure of the amount of optical power in the annealing laserbeam.

Photodetector system 60 is selected to detect laser annealing beamportion 24P at the laser (annealing) wavelength λ_(A). In the case wherelaser 20 is a CO₂ laser, an example photodetector system includes acooled HgCdTe detector. In this case, laser annealing beam portion 24Pis substantially monochromatic so that photodetector system 60 caninclude a narrow-band optical filter 61 configured to prevent thedetection of light of other wavelengths. Filter 61 may be cooled toreduce background thermal emission.

System 10 also includes a thermal emission detector system 80 thatresides along an optical axis A4 that defines an angle θ4 on wafersurface 42 relative to surface normal N. In an example, optical axis A4intersects optical axis A2 at wafer surface 42. In an example, angle θ4is equal to the Brewster's angle for the wafer. The Brewster's angle forsilicon is about 75°.

Thermal emission detector system 80 is configured to measure thermalemission radiation 82 emitted by wafer surface 42 during the laserannealing process, and generate in response thereto an electricalemission signal SE representative of the measured thermal emission E. Anexample thermal emission detector system 80, along with method ofcalculating a measured temperature T_(M) from the measured thermalemission E, is described in the aforementioned U.S. Patent ApplicationPublication No. 2012/0100640.

System 10 also includes a controller 100 that is operably connected tolaser 20, stage driver 54, photodetector system 60 and thermal emissiondetector system 80. Controller 100 has a number of logic and controlcomponents and can be configured using one or more field-programmablegate arrays (FPGAs) and other programmable and non-programmableelectronic components known to those skilled in the art, such asprocessor units, memory units, filters, feedback controllers, etc.Controller 100 is configured (e.g., via instructions embodied incomputer-readable media of one or more of the programmable components)to carry out the dual-loop control methods described herein.

Controller 100 includes a first feedback controller 112-1 shown by wayof example and referred to hereinafter as a firstproportional-integral-derivative (PID) controller. First PID controller112-1 has an input end (“input”) 113 and an output end (“output”) 114.Photodetector 60 is electrically connected to input 113, while laser 20is electrically connected to output 114.

Controller 100 also includes an emission-to-temperature logic unit (“E/Tlogic”) 120 electrically connected to thermal emission detector system80. E/T logic 120 is configured to receive the measured emission signalSE and convert the measured emission E to a measured temperature T_(M),and output a corresponding measured temperature signal ST_(M). Themeasured temperature signal ST_(M) represents an averaged measuredtemperature, as calculated from measured emission signal SE. The averageis over a time window determined by the bandwidth of thermal emissiondetector system 80 and measured emission signal SE.

The measured emission signal SE generally includes spikes due to dieemissivity variations. These spikes will show up in the measuredtemperature signal ST_(M) if they not suppressed. Consequently, in anexample embodiment, E/T logic 120 is electrically connected to alow-pass filter (LPF) 130 that low-pass filters measured temperaturesignal ST_(M) to form a low-pass filtered (“filtered”) measuredtemperature signal ST_(MF). In an example, LPF 130 is formed from asignal-processing device such as a field-programmable gate array (FPGA)that performs a running average or a spike suppression algorithm inaddition to performing low-pass filtering.

Controller 100 also includes a second feedback controller 112-2 alsoreferred to hereinafter as the second PID controller by way of example.LPF 130 is electrically connected to input 113 of second PID controller112-2. An annealing temperature set-point that is embodied in anannealing temperature set-point signal ST_(SET) is also inputted toinput 113 of second PID controller 112-2. The set-point defines aset-point annealing temperature T_(AS) for the annealing process.

The output of second PID controller 112-2 is electrically connected toinput 113 of the first PID controller 112-1. Second PID controller 112-2outputs a second control signal SC2 that corresponds to a requestedamount of laser power to maintain a substantially uniform annealingtemperature T_(A), i.e., as close to the set-point annealing temperatureT_(AS) as possible. The second control signal SC2 and the detectorsignal SD from photodetector 60 are inputted into first PID controller112-1. PID controller 112-1 processes the second control signal SC2 anddetector signal SD and outputs a first control signal SC1 to laser 20that instructs the laser to deliver a select amount of power, or toadjust its output by a select amount so that annealing laser beam 24 hasthe select amount of power.

The configuration of system 10 defines two coupled control loopsschematically indicated in FIG. 1 as L1 and L2. The first control loopL1 operates at a first frequency f₁ and is defined by laser 20,photodetector 60 and PID 112-1. The second control loop operates at asecond frequency f₂<f₁. and is defined by thermal emission detectorsystem 80, E/T logic 120, LPF 130 and PID 112-2. The two control loopsL1 and L2 are coupled at PID 112-1.

There are several sources of noise in system 10 that must be controlledto maintain a constant (uniform) annealing temperature T_(A). One noisesource is the inherent noise in the output of laser 20, which has acharacteristic 1/f distribution. Another source arises from systematiceffects within system 10 itself. One example of this type of noise isthe variation of the wafer temperature T_(W) due to non-uniformities inheated chuck 50. Another example is variation in the power density ofthe annealing laser beam 24 due to out-of-plane motion of wafer 40relative to the annealing laser beam 24, due to imperfections or setuptolerances in stage 52, which causes the power density in line image 24Lto vary during the scan.

Another source of noise is emissivity “noise” stemming from theannealing laser beam 24 passing over regions within a given die 44 thathave varying values of emissivity. This type of noise is uncompensatedin the emission feedback system, since there is no real-time, localizedmeasurement of emissivity available to correct the conversation ofemission signal to temperature. This emissivity “noise” has acharacteristic frequency that is determined by the physical spacing ofthe structures (e.g., metal lines or pads) within die 44, and the stagespeed. This constitutes “noise” in the sense that it arises from achange of the measured signal resulting from an unmeasured (and thusuncompensated) physical attribute (emissivity) of the emitting surface.

In the absence of emissivity variations, a single control loop based onemission feedback from the local annealing line image 24L can manage thecontrol of both laser 20 and systematic sources of error to maintain asubstantially constant annealing temperature T_(A). Such a system canmaintain the required power density at wafer 40 regardless if the errorbeing compensated for arises from the laser or from the system itself.

However, when the temperature measurement itself is erroneous because oflocalized changes in emissivity, such a loop will drive the system to anincorrect annealing temperature T_(A) due to the change in emissionsignal level contributed by the change in emissivity.

System 10 thus employs dual-loop control using first and second controlloops that cooperate to manage the main sources of noise, includingeliminating or greatly reducing the above-described sensitivity toemissivity “noise.”

The first control loop L1 operates to control the amount of power inannealing laser beam 24 by detecting laser annealing beam portion 24Pwith photodetector system 60 and providing the corresponding detectorsignal SD to input 113 of PID 112-1. PID 112-1 also receives the secondcontrol signal SC2 from the second control loop L2. The first loopcontrol loop L1 operates at higher frequency f₁ than the frequency f₂ ofthe second loop L2. The first control loop L1 operates at a frequency f₁sufficient to mitigate laser noise, which extends out to several hundredHz (with a 1/f roll-off). In an example, frequency f₁ is in the rangefrom about 1 KHz to about 100 KHz.

The second control loop L2 operates at a frequency f₂<f₁ and looks atthe emission from wafer 40. The low-pass filtering of measuredtemperature signal ST_(M) generates filtered temperature signal ST_(MF).This signal is an average emission/temperature with a time averagecorresponding to a spatial average (that correspondence stemming fromthe scanning stage) that spans die-scale dimensions on wafer 40. Theaveraging reduces or eliminates the adverse effects of signal spikes dueto emissivity variations. Thus, the temperature calculation is based onan average emissivity, where the “average” is taken over a relativelylarge distance (i.e., die scale). Logic in the E/T Logic 120 may also beused to facilitate suppression of emissivity-related variations in theemission signal SE, by numerical filtering, spike detection logic orother algorithms designed to mitigate the influence ofemissivity-related variations in the emission signal SE on the filteredtemperature signal ST_(MF)

The second loop L2 then controls and corrects for low frequency(relative to first frequency f₁) error components, which mainly comefrom the aforementioned systematic errors. These systematic errors havea spatial period larger than die scale and so manifest themselves (inthe time domain) at low frequency (as determined by the scan speed). Thesecond control loop L2 updates the first loop's operational set point atPID controller 112-1. In an example, second frequency f₂ of secondcontrol loop L2 is in the range from 1 Hz to 100 Hz.

Compared to the laser 1/f noise, the signal content resulting fromemissivity variations is typically more harmonic in spectral contentbecause the surface structures creating the signal variations arethemselves structured with more or less well-defined spatial periods.This results in a more or less well defined time-domain frequencyresponse determined by the stage speed and physical spacing of thestructures.

Thus, second control loop L2 measures the thermal emission E from wafersurface 42 at the location where line image 24L is being scanned. In anexample, thermal emission detector system 80 has a field of view that isat least the size of die 44 in the Y-direction (i.e., the cross-scandirection) and that is at least as wide as line image 24L in theX-direction (i.e., the scan direction). In another example, the field ofview need not cover the size of die 44 in the Y-direction, e.g., it canbe several millimeters in the Y-direction while the die can have aY-dimension of 1 cm or so. In an example, thermal emission detectorsystem 80 includes a linear array of optical fibers (not shown).

The spatial averaging of the thermal emission measurement (and hence theannealing temperature measurement) is obtained by time-averaging duringthe scanning of annealing laser beam 24 over one or more dies 44. Thus,the measurement of the thermal emission E is used to measure theannealing temperature averaged over a select area, which is translatedinto a sample time based on the speed of line image 24L, which in anexample is the speed of stage 52. In an example, thermal emissiondetector system 80 has a refresh rate of about 80 KHz that is thenaveraged by 4× to reduce the effective refresh rate to about 20 kHz.

In an example, the select area over which the emission/temperature isaveraged is the size of a die 44 or larger so that the second controlloop L2 responds to larger-than-die changes in temperature. Suchtemperature changes can arise, for example, from variations in theheating from heated chuck 50, variations in laser power density as mightbe caused by various mechanical drifts, laser efficiency/power drift,and the lower-frequency 1/f laser power variations. The second controlloop L2 operates at substantially slower frequency f₂ than the firstcontrol loop frequency f₁, such f₁>5·f₂, with an exemplary secondfrequency f₂ being about 10 Hz. In one example, f₁≈(100)·f₂.

In an example, second control loop L2 is intended to provide feedbackfor relatively low-frequency changes in the emission/temperatureresulting from more or less die-sized and up to wafer-diameter-sizechanges in substrate temperature and/or power density. If the secondcontrol loop frequency f₂ is set too close to the first control loopfrequency f₁, the second control loop will try to adjust the amount ofpower in annealing laser beam 24 based on small changes in emission dueto local changes in emissivity, e.g., the emissivity changes within asingle die 44. It has been found that the annealing process is improvedwhen such small-scale emissivity changes are ignored and the amount ofpower in annealing laser beam 24 is adjusted based on relativelylarge-area average emissivity (e.g., a die-size area, a multiple-diesize area, etc.).

Thermal emission detector system 80 can include one or more detectors81, including multiple detectors operating at multiple wavelengths formulti-color pyrometry. The one or more detectors preferably havesufficient detectivity (D*) to yield a reasonable signal-to-noise ratio(SNR) for the photon flux associated with thermal emission radiation 82at the set-point annealing temperature T_(AS) and for the givenannealing laser beam geometry. An example thermal emission detectorsystem 80 operates over a band-limited wavelength range for which theband-integrated thermal emission has good sensitivity to changes intemperature about the set-point annealing temperature T_(AS).

For example, in the case of laser annealing junctions, the laserannealing temperature T_(A) is in the range 1100° C. to 1250° C., andthe appropriate wavelength range for thermal emission radiation is 500nm to 900 nm. At these wavelengths, photo-multiplier tubes or Si-basedphotodiodes can be used as detectors. For contact annealing, the laserannealing temperatures T_(A) are lower, e.g., in the range 800° C. to1000° C., and a longer wavelength detector sensitive in the 1 μm to 2 μmregion is appropriate, such as an InGaAs detector.

While the first and second control loops L1 and L2 are physicallycoupled and serve to adjust the laser power, the aforementioneddifference in control loop frequencies f₁ and f₂ serve to functionallydecouple the two control loops so that each can operate in independentlyand in a stable manner, so that one control loop does not introduceinstabilities into the other.

The dual-loop control approach to laser annealing disclosed herein canbe considered a hybrid approach based on wafer-to-wafer (WTW) andwafer-within-wafer (WIW) approaches. The second control loop L2 providesWIW-like feedback, but operates at a relatively “slow” frequency f₂ sothat it is averaging emission over one or more dies 44, while the firstcontrol loop L1 is “fast” to compensate for corresponding 1/f noise inlaser 20 that contributes to unwanted power fluctuations and thusunwanted annealing temperature variations.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present disclosurewithout departing from the spirit and scope of the disclosure. Thus itis intended that the present disclosure cover the modifications andvariations of this disclosure provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A laser annealing system for annealing diessupported by a wafer having melt temperature and a surface that supportsdies each having a variation in emissivity, comprising: a chuck thatsupports the wafer, and a movable stage that supports the chuck and thatis adapted to move the chuck and wafer; a laser that generates aninitial laser beam and that has an adjustable output power and an amountof 1/f noise that can cause variations in an annealing temperature,wherein the annealing temperature is less than the wafer melttemperature; an optical system configured to receive the initial laserbeam and form therefrom an annealing laser beam and to direct a portionof the annealing laser beam to a photodetector system that in responsegenerates a detector signal, the optical system being arranged so thatthe annealing laser beam is made incident upon and scans over the wafersurface due to the movement of the wafer, and heats the wafer to theannealing temperature; a thermal emission detector system arrangedrelative to the wafer surface and configured to receive thermal emissionradiation therefrom that is generated by the annealing laser beam andaverage the received thermal emission radiation over an area of one ormore of the dies, and in response generates a thermal emission signal; afirst control loop configured to receive the detector signal and controlthe laser to adjust the output power at a first frequency f₁ in therange from 1 kHz to 100 kHz to reduce the amount of 1/f noise; and asecond control loop configured to receive the thermal emission signaland control the laser to adjust the output power at a second frequencyf₂ in the range from 1 Hz to 100 Hz to reduce variations in theannealing temperature.
 2. The system of claim 1, wherein the annealingtemperature is in the range from 400° C. to 1350° C.
 3. The system ofclaim 1, wherein the thermal emission signal includes spikes, whereinthe thermal emission detector system includes a filter, and wherein thethermal emission signal is passed through the filter to reduce oreliminate the spikes.
 4. The system of claim 1, wherein the first andsecond control loops respectively include first and secondproportional-integral-derivative controllers that are coupled to oneanother.
 5. A dual-loop control system for a laser annealing system thatuses an annealing laser beam from a laser having 1/f noise and acontrollable output power to anneal a wafer having a melt temperatureand dies that each have a variation in emissivity, comprising: a firstcontrol loop operating at first frequency f₁ in a range between 1 kHzand 100 kHz and configured to control the laser output power to reducethe 1/f laser noise that can cause variations in an annealingtemperature, wherein the annealing temperature is less than the wafermelt temperature; and a second control loop operating at a secondfrequency f₂ in a range between 1 Hz and 100 Hz and that measuresthermal emission radiation from the wafer over an area of one die orgreater, the thermal emission being caused by the annealing laser beamheating the wafer to the annealing temperature, the second control loopbeing configured to determine a measured wafer temperature from themeasured thermal emission radiation and generate a measured temperaturesignal, which is used in combination with an annealing temperature setpoint signal to control the laser output to reduce variations in theannealing temperature.
 6. The dual-loop control system of claim 5,wherein the second control loop includes a thermal emission detectorsystem that measures the thermal emission within one or more dies andthen averages the within-die measurements to said area of one die orgreater.
 7. The dual-loop control system of claim 5, wherein the firstcontrol loop includes a photodetector system that receives a portion ofthe annealing beam created by a beam-turning element, the photodetectorsystem generating in response a detector signal representative of anamount power in the annealing laser beam, and wherein the first controlloop includes a first proportional-integral-derivative (PID) controllerthat receives the detector signal as well as a second control signalfrom the second control loop and that generates a first control signalin response thereto and sends the first control signal to the laser. 8.The dual-loop control system of claim 7, wherein the second control loopincludes a second PID controller that receives the measured wafertemperature signal and the annealing temperature set-point and inresponse generates the second control signal.
 9. The dual-loop controlsystem of claim 5, wherein the second control loop includes emission totemperature (E/T) logic that calculates the measured temperature fromthe measured emission.
 10. The dual-loop control system of claim 9,wherein the second control loop includes a low-pass filter that filtersthe measured temperature signal from the E/T logic prior to inputtingthe measured temperature signal to the second PID controller.
 11. Alaser annealing system for performing laser annealing of a wafer havinga melt temperature and a surface that supports an array of dies witheach die having a varying emissivity, comprising: means for scanning anannealing laser beam from a laser over the array of dies, wherein thelaser has an amount of laser noise and is adjustable to control anamount of power in the annealing laser beam, wherein the annealing laserbeam creates an annealing temperature at the wafer surface that does notexceed the wafer melt temperature; means for measuring and controllingthe amount of power in the annealing laser beam using a first controlloop that measures the amount of power and operates at a first frequencyf₁ to control the amount of laser noise in the laser; and means forcontrolling the amount of power in the annealing laser beam using asecond control loop that operates at a second frequency f₂<f₁ bymeasuring thermal emission radiation from the wafer, including averagingthe thermal emission radiation over at least one die and determiningtherefrom a corresponding average measured temperature, and using theaverage measured temperature and an annealing temperature set point toadjust the laser to control the amount of power in the annealing laserbeam.
 12. The system of claim 11, wherein the first frequency f₁ is in arange from 1 kHz to 100 kHz and the second frequency f₂ is in the rangefrom 1 Hz to 100 Hz.
 13. The system of claim 11, wherein f₁ is about(100)·f₂.
 14. The system of claim 11, wherein measuring the amount ofpower in the laser annealing beam includes deflecting a portion of thelaser annealing beam to a photodetector system.
 15. The system of claim14, wherein deflecting a portion of the laser annealing beam includesdiffracting a portion of the laser annealing beam using a grating formedon a reflective surface from which the laser annealing beam otherwisereflects.
 16. The system of claim 11, wherein the system is configuredsuch that the step of measuring the thermal emission radiation from thewafer is performed at Brewster's angle.
 17. The system of claim 11,wherein the first and second control loops respectively employ first andsecond proportional-integral-derivative controllers that are coupled toone another.
 18. The system of claim 11, wherein the laser annealingbeam creates an annealing temperature at the wafer surface in the rangefrom 400° C. to 1350° C.